Hi I have a 2000 4.0 that loses oil pressure once it gets warm at idle. Still builds pressure fine with RPM increase but never really goes above 40. I know these cherokees run about 40-60 at around 65 mph.

So I replaced the oil pump and checked the rod and main bearings, and when I pulled them all down they looked really good, so I didn't replace them.

What would be the next thing to check? Would it be the cam bearings?

 

What kind of oil filter do you have on it. A cheap filter such as Fram and any off brand can cause low pressure. If you have any of those cheap filters on it put a Mopar, Wix or Napa Gold. Also you can verify the pressure with a mechanical pressure gauge and see if it's close to what you are seeing on the dash.

 

It's got a FRAM on it, I'm changing the oil tomorrow and putting a Mopar on. Do I have to buy a mechanic guage or can I rent one from a parts store?

 

Also, I'm going to be very upset if I put this guage on tomorrow and it has good pressure, because I replaced the sending unit, however the one I replaced it with was a used one sitting in a box in the garage :P

 

You can rent one at any Auto Zone or that type of parts store. BTW, Minimum pressure is 13psi at idle for our 4.0 so if you are in that area you will be fine.

 

oh i can imagine how pissed off you will be if the oil filter was the culprit

 

Pumps don't make pressure, they only make the oil flow. It's the bearing and lifter clearances the make the pressure. Too much clearance and you will have lower oil pressure.

 

I don't suppose you measured the clearance with plastigauge, was it within spec?

 

Yeah I used plastigauge on the main and rod bearings, they were all well within the limit. Picked up the mopar filter and some full synthetic syn power valvoline for 300k miles, we'll see what happens tomorrow, and i'm gonna grab that mechanical gauge from autozone

 

Year 2000, thats says it all. Fairly common problem that year. My 2000 engine has 4psi at idle and 20 at highway speed. I would suspect a cam bearing. I would not be too concerned b/c mine has had LOWER pressure than yours for years with no issues.

 

I would try an experiment since it's got fresh oil in it. Change JUST the filter to one of those recommended by RTorrez and see what happens. Good experiment in the meantime.

 

x2 but don't get your hopes up. I went down the same path years ago with no success and no definitive answer on what was causing low pressure. I decided to drive the he!! of it until it blows but it continues to run like a sewing machine!

 

mine did the same thing. NAPA gold filter fixed it.

 

I ended up getting called into work so I will have to change the oil tomorrow instead of today, but the crappy thing is my dad has been driving cherokee's for ages on his us post office route and using fram filters with no problems, then again he's never had one newer than 99

I expect the filter to fix it but it just makes me mad that a filter is what was probably causing the problem LOL.

Good news is I got my money back for the rod bearings

 

Try JUST the filter for the heck of it.